Researchers at the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) have described the damage mechanism of calcium fluoride (CaF₂) crystal, an important component in the ultraviolet ray (UV) optical system, and concluded that it's closely related to cleavage planes and sliding systems. Related results were published in Applied Surface Science.

Led by Prof. LIANG Xu and Prof. SHAO Jingzhen, the researchers studied laser-induced damage characteristics using a self-developed krypton fluoride excimer laser and analyzed the formation mechanism of three different crystal surface morphologies.  

They investigated the cleavage plane {111}, the slip system {100} <110>, the angular relationship between the crystal plane, the slip system and cleavage plane, and found it's the structural characteristics of the CaF₂ crystal that led to the different mechanical properties on different crystal planes.

CaF₂ crystal directly affects the stability and life of UV optical system. Therefore, scientists have turned their attention to the process and mechanism of the interaction between UV laser and the CaF₂ crystal to improve its laser damage resistance.

In this study, the researchers completed laser irradiation experiment on CaF₂ crystal with this 248 nm excimer laser. They obtained the evolution of damage morphology and the laser-induced damage thresholds of three different CaF₂ crystals planes (100), (110) and (111).

The results showed different damage patterns. The damage of plane (111) shows the phenomenon of lamellar stripping, and there is burst-like damage on plane (110) and plane (100), which is caused by cracks or local damage points.  

These results pave the way for the application of CaF₂ crystals with different crystal planes in UV laser system.

This work was supported by the Scientific Equipment Development Project of CAS, the Youth Innovation Promotion Association of CAS, the National Natural Science Foundation of China, and the Open Research Fund of the State Key Laboratory of Pulsed Power Laser Technology.

The laser-induced damage morphology of the CaF₂ crystal depends on the crystal structure. (Image by SHAO Jingzhen)